Gated ultrasound imaging apparatus and method

ABSTRACT

An ultrasound imaging system is programmed to acquire first ultrasonic image frames intermittently. These first frames, typically triggered frames synchronized with a selected portion of an ECG cycle, are optimized for high image quality of a contrast agent included in the tissue. The imaging system automatically acquires second ultrasonic image frames between at least some of the first frames. The second image frames are typically locator frames which are optimized for reduced degradation of the contrast agent. More of the second frames are acquired per unit time than first frames, and both the first and second frames are displayed, either superimposed over one another or in side-by-side relationship. In this way the user is provided with substantially continuous transducer locating information, yet contrast agent destruction between acquisitions of the first, triggered frames is reduced or eliminated.

This application is a division of application Ser. No. 09/378,236, filedAug. 19, 1999, , which is a continuation of Ser. No. 08/838,919 now U.S.Pat. No. 6,112,120 (filed Apr. 11, 1997) and Ser. No. 09/159,527, nowU.S. Pat. No. 5,957,845 filed Sep. 23, 1998.

BACKGROUND OF THE INVENTION

The present invention relates generally to ultrasound imaging systems,and more specifically to ultrasound imaging systems which provideimproved visualization of contrast agents.

Ultrasound imaging systems usually are operated in a fashion to producereal-time moving images of a subject being scanned. These moving imagesare acquired as discrete static images, but at a high enough frame rate(typically 20-30 frames/sec) to present the illusion of a continuouslymoving image. Commercial ultrasound systems have also included triggeredacquisition modes. In these modes, an ultrasound image frame is acquiredat a specified point in each cardiac cycle, as measured for example by adelay from the R-wave of an ECG waveform. Typically the ultrasoundsystem is quiescent between acquisition of successive triggered frames,neither transmitting nor receiving, and the system display is static,showing the last triggered frame. For example, an ultrasound system canbe programmed to generate a triggered frame at 100 ms after each R-wave.At typical human heart rates of 60-120 beats/minute, this results in theimage being updated at 1-2 frames/second, rather than the 20 or moreframes/sec that might be possible if scanning were continuous. In othervariants, triggered frames may be acquired only on selected beats (e.g.,150 ms after every 3rd R-wave), or multiple frames may be acquired perbeat (e.g., 100, 150, and 250 ms after every 2nd R-wave).

One application where gated imaging modes are useful is imaging ofultrasound contrast agents. Contrast agents are injected into thebloodstream to increase the brightness of blood and blood-perfusedtissues. However, these contrast agents (which are typically composed ofstabilized gas microbubbles a few microns in diameter) are fragile andeasily degraded (destroyed or altered) by the ultrasound pulses used toimage them. A first ultrasound frame may show the contrast agent well,but subsequent frames often show less and less signal as the contrastagent is destroyed.

Thomas Porter and other researchers have demonstrated that gated imagingmay be used to advantage where bubble destruction is an issue. A singleimage frame is acquired every cardiac cycle (or every few cardiaccycles). During the interval between frame acquisition, while theultrasound transmitters are inactive, fresh contrast agent circulatesinto the tissues and vessels being imaged. Thus, if the interval betweensuccessive triggered frames is sufficiently long, a second acquiredframe presents a signal that is as strong as the first. Some researchershave proposed that bubbles are not destroyed by ultrasound, but arealtered in some way; during the interval between frames, the bubbles mayrecover in some way. In this case, the effect is the same: after aninterval without transmission, the image returns to its initialbrightness.

Another method used to reduce bubble destruction is to transmit at areduced ultrasound intensity. This reduces bubble destruction at thecost of a reduced signal-to-noise ratio.

Another property of contrast agents that should be mentioned isnon-linear scattering. Many contrast agents, when insonified with anacoustic pulse centered at one frequency, reflect or radiate ultrasoundcontaining components at harmonics of the insonifying frequency as wellas at the insonifying frequency. This property has been used toadvantage in distinguishing contrast agents from normal tissues, whichdo not tend to scatter non-linearly. U.S. Pat. No. 5,255,683 (Monaghan),U.S. Pat. No. 5,410,516 (Uhlendorf), U.S. Pat. No. 5,456,257 (Johnson)and U.S. Pat. No. 5,577,505 (Brock-Fisher) disclose techniques forimaging non-linear scattering from tissues, and several ultrasoundmanufacturers are known to be developing second harmonic imagingcapability (that is, forming an image from energy scattered at aharmonic multiple of twice the insonifying frequency). Harmonic imagingsuffers from the same bubble destruction effects as does conventionalfundamental imaging, and the same techniques of gating and reducedtransmit power may be used.

Conventional gated imaging techniques require a user to hold anultrasound probe in a fixed location for as long as several heartbeatswithout any visual feedback from the image, which statically show thepreviously acquired frame. Furthermore, static images of dynamicstructures such as the heart may be difficult to interpret and maycontain less diagnostic information than moving images. Reducingtransmit power reduces bubble destruction, but makes the image noisierand ultimately limits the penetration depth (the maximum depth that maysuccessfully be scanned). Transmit power reductions of 15 dB or more(below the maximum attainable by a typical diagnostic system under FDAlimitations) may be required in order to avoid destroying bubbles whenimaging perfusion of tissues. Transmit power reduction may beparticularly disadvantageous in harmonic imaging. The harmonic componentof the scattered and received signal is typically much smaller than thefundamental component (only a small fraction of the incident acousticenergy is converted to higher harmonic frequencies), while the noisefloor remains roughly constant. Further, because of the non-linearityinherent in generating higher-order harmonics, a given reduction intransmit power results in an even greater reduction in the harmonicsignal-strength. For example, a 3 dB transmit power reduction may resultin roughly a 6 dB decrease in the level of the second harmonic signal.

SUMMARY OF THE INVENTION

The present invention is directed to an ultrasound imaging method andapparatus as defined by the following independent claims.

One preferred embodiment of the invention includes an ultrasoundbeamformer which acquires harmonic triggered frames using high transmitpower responsive to a trigger signal derived from an ECG waveform.During the interval between acquisition of these triggered frames,additional image frames are acquired at reduced transmit power. Ingeneral, the in-between frames (referred to as “locator frames”) areoptimized for low levels of bubble degradation, possibly at the expenseof image quality, while the triggered frames (also referred to as“imaging frames”) are optimized primarily for image quality. Fundamentalimaging is often preferred for the locator frames because it has asignificantly higher signal-to-noise ratio than harmonic imaging, andhence can generate a usable image at much lower transmit power levelsthan would be required for useful harmonic imaging. These locator framesare displayed in real-time on a display device, providing the user withcontinuous feedback as to the location of the scan plane. The triggeredframes may be displayed in real-time along with the locator frames in avariety of ways as described below, or may be reviewed later from memorywith or without the locator frames.

In a broad sense, the invention includes any technique for alternatingbetween two types of frames, one adapted to obtain a high-quality imageof tissues containing contrast media and triggered intermittently, and asecond adapted not to destroy the bubbles imaged by the first frame.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an ultrasound imaging system thatincorporates preferred embodiments of the present invention.

FIG. 2 is a flow chart illustrating operation of the beamformercontroller of FIG. 1.

FIG. 3 is a flow chart providing further details regarding operation ofthe beamformer controller of FIG. 1.

FIG. 4 is a timing diagram illustrating a first mode of operation of theimaging system of FIG. 1.

FIGS. 5a-5 e are frequency diagrams of transmit and receive spectra foralternative modes of operation of the imaging system of FIG. 1.

FIG. 6 is a schematic diagram of a display shown on the display deviceof FIG. 1.

FIGS. 7 and 8 are timing diagrams for two additional modes of operationof the imaging system of FIG. 1.

FIG. 9 is a flowchart showing a mode of operation of the controller 16for acquiring sequences of triggered ultrasonic image frames.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Turning now to the drawings, FIG. 1 provides a schematic diagram of anultrasound imaging system 10 that incorporates a presently preferredembodiment of this invention. The imaging system 10 includes aprogrammable transmit/receive beamformer system 12 that is coupled to anultrasonic transducer 14. The beamformer system 12 provides transmitwaveforms to the transducer 14 which cause the transducer 14 to emitultrasonic energy into a tissue T containing a contrast agent C.Scattering sites within the tissue T, including the contrast agent C,return ultrasonic energy to the transducer 14, which transmits receivewaveforms to the beamformer system 12. The region from which receivewaveforms are collected will be referred to as the imaged region, andmay include tissue, blood, and optionally contrast agent.

A beamformer controller 16 controls operation of the beamformer system12 by controlling beamformer parameters such as transmit centerfrequency and bandwidth, receive center frequency and bandwidth,transmit power, receive gain, and transmit line spacing. The beamformercontroller 16 is responsive to user controls 22 and to a trigger signalsource 18. The trigger signal source 18 is responsive to an ECG signalsupplied by an ECG device 20, and the trigger signal source 18preferably includes conventional software or hardware which recognizesthe R-wave of an ECG signal and generates a trigger signal when eachR-wave occurs. The trigger signal is used as discussed below by thebeamformer controller 16 to select appropriate beamformer parameters.

Detected, formed receive beams from the beamformer system 12 are sent toa display controller 24 which preferably includes a scan converter andgenerates triggered and locator frames as described below for immediatedisplay on a display device 26. The beamformer system 12 is also coupledto a cine memory 28 which stores triggered and locator frames for laterplayback. The display controller 24 is capable of displayingsuperimposed locator frames and triggered frames on the same area of thescreen of the display device 26 as they are acquired. Alternatively,locator frames may be displayed on different areas of the display screenfrom the triggered frames, as discussed below in conjunction with FIG.6.

The present invention is directed to the structure and operation of thebeamformer controller 16, and all of the remaining elements of FIG. 1can be formed in any suitable manner, including a wide variety ofconventional systems. The widest variety of trigger signal sources,beamformer controllers, beamformer systems, transducers, displaycontrollers, displays, and cine memories can be adapted for use withthis invention. Both analog and digital beamformer systems are suitable,and a wide variety of signals can be provided as inputs to the triggersignal source. By way of example, without intending any limitation, theultrasound imaging system marketed by Acuson Corporation under the tradename Sequoia is capable of being modified to implement this invention.

A flow chart illustrating one mode of operation of the beamformercontroller 16 of FIG. 1 is given as FIG. 2. In this mode, the userselects a count of R-waves N and a programmable trigger delay ΔT. Acounter M is initially set to 0. The controller 16 counts R-waves untilN R-waves have been detected, then initializes a timer t. When theprogrammed interval ΔT elapses without interruption by an R-wave, thecontroller suspends acquisition of any further locator frames aftercompletion of any frame acquisition in progress. After locator frameacquisition has been completed, the controller changes beamformerparameters (setting the transmit power to maximum and selecting aharmonic receive center frequency), and acquires a single triggeredframe. Following acquisition of the triggered frame, the controllerchanges beamformer parameters again (reducing the transmit power andselecting a fundamental receive center frequency) and resumes continuousfiring of locator frames. The process then begins anew. The latter threesteps of suspending locator frame acquisition, acquiring a triggeredframe, and resuming locator frame acquisition are shown in more detailin FIG. 3.

As used herein, “harmonic” is intended broadly to include subharmonicand fractional harmonic energy (e.g. 1/2 or 3/2 of the fundamentalfrequency at which the transmit beam is centered) as well as higherharmonics (e.g. 2 or 3 times the fundamental frequency at which thetransmit beam is centered). In addition, a harmonic image signal orframe may be acquired from a single frame of scan lines that are eachfired once, or alternately a harmonic image signal or frame may beprocessed from multiple frames or from frames where individual scanlines are fired multiple times. See, for example, the harmonic imagesformed with subtractive processing in the above-identified Johnson andBrock-Fisher patents.

Of course, various modifications to the flowcharts of FIGS. 2 and 3present themselves, and these flowcharts should be taken as exemplary ofone possible implementation of the invention. In particular, theflowchart of FIG. 2 shows one way of counting R-waves and measuring timedelays; various other ways of counting and measuring are possible andcan be substituted. The behavior of the system when the time comes toacquire a triggered frame can also be modified. Instead of waiting foran ongoing locator frame to be acquired, the controller can abort thelocator frame in progress, discarding the partially acquired frame, soas to be able to begin the acquisition of the triggered frame as closeto the desired time as possible, For cases where the trigger delay ΔTbetween R-wave and triggered frame is greater than the time required toacquire a locator frame, the controller may anticipate a comingtriggered frame acquisition and suspend locator frame acquisition beforethe trigger delay ΔT has elapsed.

The operation of the system of FIG. 1 is illustrated schematically inFIG. 4. An ECG waveform 30 is shown across the bottom of the figure.Within the trigger signal source 18, conventional software or circuitrydetects the peak of the R-wave, as illustrated by the vertical lines 32above the ECG waveform 30. In this example, the interval between R-wavesis 750 ms (heart rate=80 bpm) and the trigger signal source 18 generatesa trigger signal 150 ms after every second R-wave (N=2, ΔT=150 ms). Ateach trigger, acquisition of locator frames is suspended, and a singletriggered frame is acquired using alternate beamforming parameters suchas those suggested above (harmonic imaging; high transmit power).Following acquisition of the triggered frame, acquisition of locatorframes resumes using the appropriate beamformer parameters (fundamentalimaging; low transmit power). In this implementation, both locatorframes and imaging frames are directed to the screen as they areacquired, forming an apparently continuous image (with a flicker as eachtriggered frame is acquired). On later cine review, the triggered framesmay be distinguished from the locator frames. Alternately, locatorframes may be excluded from later cine review. Additionally the locatorand image frames may be combined (optionally color-coding one image thenadding the two together) into a single image. The user would then see asuperposition of a relatively static image (the triggered frames) with amore dynamic one (the locator frames).

In addition to changing transmit power and selecting fundamental vs.harmonic imaging, the transmit center frequency, spectral shape, and/orbandwidth can be different for the locator and triggered frames.Contrast agents increase sound scattering through a resonancephenomenon, and the center frequency of that resonance varies inverselywith bubble size. Bubbles of a given size scatter more energy (and mayproduce higher levels of harmonics) at or near the correspondingresonance frequency. At the same time, bubbles are more likely to bedestroyed by ultrasound at or near their resonance frequency than byultrasound away from their resonance frequency. Contrast agents achievea broad bandwidth of contrast enhancement partly because each agent asinjected includes many bubbles of different sizes and hence differentresonance frequencies,

FIGS. 5a- 5 d show how these properties of contrast agents may be usedadvantageously to reduce bubble destruction. FIG. 5a shows one preferredtransmit spectrum for the triggered frames, centered at 2.5 MHz. Thetriggered frames may be acquired in fundamental or harmonic imaging (inwhich case the receive spectrum would be as shown by the dashed linecurve); in either case, the greatest contribution to the image comesfrom bubbles with resonance near 2.5 MHz. FIG. 5b shows a preferredtransmit spectrum of locator frames, centered (for example) at 4.0 MHz.While the locator frames may be optimized to minimize bubbledestruction, some bubbles may be destroyed anyway. However, thedestroyed bubbles will tend to have a resonance frequency at or near 4.0MHz. Thus the bubbles destroyed by the locator frames are not thosebubbles primarily imaged by the triggered frames. Additional benefit maybe derived in this example from the fact that higher frequencies maytend to destroy bubbles to a lesser extent than lower frequencies.

In general, the beamformer controller 16 of FIG. 1 may control thetransmit waveform of the beamformer system 12 during locator frameacquisition to minimize or eliminate ultrasonic energy transmitted atand near the fundamental frequency f_(o) of the triggered frames. Analogfilters, pulse shapers, or digital filters may be used in the transmitbeamformer to reduce ultrasonic energy transmitted at or near thefundamental frequency of the triggered frames. Preferably, theultrasonic energy level at the fundamental frequency f_(o) for eachtransmit pulse during locator frame acquisition is at least 6 dB, morepreferably at least 12 dB or 20 dB, and most preferably at least 30 dB,below the ultrasonic energy level at the same frequency during triggeredframe acquisition. In the preferred embodiment of FIG. 5c, the transmitwaveforms are positioned in frequency to ensure that the spectrum 40 ofthe transmit waveforms for the locator frames has substantially noenergy in a frequency band centered on the transmit frequency f_(o) ofthe triggered frames. This band preferably extends on both sides off_(o) over a frequency range of one-tenth (more-preferably one-fifth) off_(o). Alternately, the band may extend on both sides of f_(o) tofrequencies at which the transmit waveforms for the triggered frameshave an amplitude at least 6 dB below the amplitude at f_(o). In thisembodiment, the spectrum 42 of the transmit waveforms for the triggeredframes has its peak energy level at f_(o) (FIG. 5d). Of course, it isnot critical that the spectrum 40 be centered at 2f_(o) as shown in FIG.5c, and other center frequencies can readily be chosen. In the preferredembodiment of FIG. 5e, the spectrum 44 of the transmit waveform for thelocator frames has been shaped or filtered in the region under thedotted line to substantially eliminate ultrasonic energy in a bandcentered on the transmit center frequency f_(o) of the triggered frames.

Several different means of displaying the triggered and locator framesare possible. FIG. 6 shows one arrangement, in which the locator frames34 are shown on a spatially separated region of the screen from thetriggered frames 36. In the example, the locator frames 34 are shown asa small moving image situated apart from the relatively static (updatedonce every several cardiac cycles) triggered image 36.

An alternate mode of operation is illustrated in FIG. 7. In this case,locator frames are suspended when each triggered frame is fired, and fora selected period after acquisition of the triggered frame is completed.In FIG. 7, the total duration of suspension of the locator frames is 300ms. The locator and triggered frames are displayed on the same area ofthe screen, so that the user sees a moving image (the locator frames)which periodically stops briefly (the triggered frames, each held for apersistence interval of 300 ms). In variations on this approach, thetriggered or locator frames can be made more easily distinguishable fromone another by changing their brightness or by color coding. In anothervariation, locator frames may be acquired during the hold period (oncethe triggered frame acquisition is complete), but not displayed. Theseframes are then available for later review. FIG. 7 also illustrates amultiple trigger mode of operation, which may be used independently ofthe persistence interval feature discussed above. The multiple triggermode of operation is discussed below.

The embodiments described above improve upon the technique of usingintermittent scanning to image fragile contrast agents by displaying amoving image even when the interval between triggered frame acquisitionis large. This greatly eases the operator task of trying to hold theprobe in a fixed position relative to the tissues being imaged in orderto maintain a constant and appropriate scan plane. The locator framesare preferably formed using very low transmit power and so do notsignificantly destroy the contrast agent being imaged. While thetriggered frames may destroy the contrast bubbles, the interval betweentriggered frames is large enough for the tissues being imaged to berefreshed with new contrast agent.

A further advantage of these embodiments is that a single ultrasoundframe (the triggered frame) may be easier to interpret using the dynamiclocator frames as context.

Many alternate methods of construction are possible. For example, thelocator frames may be acquired at an artificially reduced frame rate.For example, if acquisition of a frame normally takes 25 ms (40frames/sec), the locator frames can be acquired at 100 ms intervals (10frames/sec) by adding dead time between adjacent scan lines or frames,further decreasing bubble destruction associated with acquisition of thelocator frames. Even in this case, there are preferably more locatorframes than triggered frames per unit time. Also, one or both of thetriggered and locator frames may be acquired using a reduced linedensity. Reduced line density (with subsequent loss of resolution andloss of artifact) may be acceptable for the locator frames, and can beadvantageous if used with an artificially reduced frame rate so that theframe rate is not increased as the line density is reduced. In thiscase, the reduced line density results in a reduced level of totalultrasound energy delivered into the subject being imaged, and hence inreduced bubble destruction. Reductions in line density may be beneficialfor the triggered frames, as overly high line densities may result inexcessive overlap of the transmit beams, resulting in bubble destruction(one ultrasound line firing destroying bubbles which would otherwisecontribute to imaging of an adjacent line).

Another alternative is that triggered frames may be acquired usingfundamental imaging instead of harmonic imaging. The locator framesshould be optimized to reduce degradation of the bubbles imaged in thetriggered frames. The locator frames may be acquired using differenttransmit center frequency, bandwidth, and/or pulse shape than thetriggered frames. In general, any alteration in the transmitcharacteristics of the locator frames which results in less destructionof the bubbles preferably imaged in the triggered frames may beadvantageous. In particular, using a different transmit center frequencymay result in the triggering frames selectively destroying a populationof bubbles different from those primarily contributing to the triggeredframe images. In some embodiments, this observation may be exploited tofullest advantage by the use of hardware or software filtering means inthe transmit beamformer to remove any components at the triggered frametransmit frequency from the locator frame transmit pulses. One or bothof the locator frames and triggered frames can be acquired using achirp, swept-spectrum, coded excitation, or other high time-bandwidthproduct transmit pulse. Such signals may attain a given signal-to-noiseratio with lower peak pressures than a conventional ultrasound pulse,and hence may provide better performance for a given level of bubbledestruction. In some implementations, such techniques may degrade imagequality by worsening axial response or by worsening focusing. Suchtradeoffs may be acceptable for locator frames but not for triggeredframes. In the event that such transmit signals are used, the receivebeamformer should include means to restore the axial resolution as wellas possible.

Instead of acquiring only a single triggered image frame, a selectablenumber of frames can be acquired in quick succession (continuous frameacquisition). Various complex trigger schemes can be used to determinewhen to acquire triggered frames. As a first example, multipleindependently selectable trigger delays can be selected. An example isshown in FIG. 8, where two closely spaced triggered frames are acquired,300 and 450 ms after every 3rd R-wave. In this example, the controlleris programmed not to fire any locator frames during the time intervalbetween the two triggered frames (which occur fairly close together).Trigger delays can be varied following each triggered frame to obtainsequences of triggered frames at various points in the cardiac cycle(“swept triggers”), as shown in FIG. 7. As an example, the firsttriggered frame is acquired immediately following the first R-wave.Subsequent triggered frames are acquired at 150, 300, 450, and 600 msafter the second through fifth R-waves, respectively. The cycle thenrepeats itself following the seventh R-wave. As a second example,triggered frames can be acquired at different time intervals afterdifferent R-waves; for example at 20 ms after every 1st, 4th, 7th, . . .R-wave; and at 500 ms after every 2nd, 5th, 8th . . . R-wave.

Various alternatives to ECG R-wave detection are possible: triggeringcan be based on a different feature of an ECG signal; an externallyprovided trigger signal; some other physiological measured signal suchas respiration; or on a combination of signals (such as triggering onthe first R-wave after the peak of a measured respiration signal, so asto compensate for breathing).

Another aspect of the invention relates to the acquisition of multipletriggered frames, even in the absence of locator frames. For example, inthe foregoing discussion two triggered frames are acquired after everythird R-wave. FIG. 9 shows a flowchart for another mode of operation ofthe beamformer controller 16. In this mode, the user selects a count ofR-waves N and two programmable trigger delays ΔT1 and ΔT2, with ΔT1 lessthan ΔT2. A counter M is initially set to zero. The con roller 16 countsR-waves until N R-waves have been detected, then initializes a timer T.When the first programmed interval ΔT1 elapses without interruption byan R-wave, the controller acquires a first triggered frame. When thetotal interval ΔT2 has elapsed since the timer was initialized (that isan additional time interval of ΔT2−ΔT1 after the first frame wasacquired), the controller acquires a second triggered frame. The R-wavecounter is then reset, and the entire process repeats. In this example,N is greater than 2.

This mode of operation may be generalized so that more than two framesare acquired, each specified by a respective time delay.

In an alternate mode of operation, after the first triggered frame isacquired, the counter may count a second number of R-waves N2, and asecond time delay ΔT2 before acquiring the second triggered frame.Again, this mode of operation may be generalized to more than two framesper sequence. For example, in the swept trigger sequence of FIG. 7,triggered frames are acquired with five different trigger delays afterfive respective R-waves. Following the fifth acquisition, the controllercounts two R-waves before beginning the sequence anew.

In general, the R-wave can be considered as one example of a markersignal that occurs at a specified portion of an ECG wave. This aspect ofthe invention generates a marker signal such as an R-wave signal at aspecified portion of a plurality of cycles of an ECG wave. Then asequence of a plurality of triggered ultrasonic image frames isacquired, each frame timed to follow a respective marker signal by arespective time interval. In some cases two or more triggered ultrasonicimage frames may share the same marker signal, in which case multipleones of the triggered ultrasonic image frames will occur within a singlecycle of the ECG wave. Following acquisition of this sequence oftriggered ultrasonic image frames, acquisition of image frames isinterrupted for at least one cycle of the ECG wave (two marker signals),before a next sequence of image frames is acquired.

As used herein, the term “responsive to” is intended broadly to coverany situation where a first element alters its operation in response toa signal generated by a second element, whether directly or indirectly.Thus, the first element is said to be responsive to the second when thefirst element responds directly to an output signal of the secondelement. Similarly, the first element is responsive to the second ifintermediate elements or processors alter or modify a signal of thesecond element before it is applied as an input to the first element.

Many alternate methods of construction or use of the invention will beobvious to one skilled in the art, and the invention should not belimited to the specific examples or combinations discussed above. It isonly the following claims, including all equivalents, which are intendedto define the scope of this invention.

We claim:
 1. In an ultrasound imaging system comprising: a programmabletransmit/receive beamformer system operative to generate transmitwaveforms and responsive to ultrasonic signals radiated by a region of abody, the improvement comprising: a trigger signal source; and abeamformer controller coupled with the trigger signal source and thebeamformer system, said controller operative to cause the beamformersystem to acquire multiple triggered frames at different intervals, afirst of the triggered frames acquired after a first count of signalperiods from the trigger signal source and a second of the triggeredframes acquired after a second count of signal periods from the triggersignal source, the second count being started after the first count. 2.The system of claim 1 wherein the trigger signal source is responsive toan ECG signal.
 3. The system of claim 2 wherein the signal periodscomprise heart cycles and the first and second counts differ by anintegral number of heart cycles.
 4. The system of claim 1 wherein thedifferent intervals are programmed.
 5. The system of claim 4 furthercomprising a user interface for programming the different intervals byselection of the first and second counts.
 6. The system of claim 1wherein the beamformer controller automatically programs said beamformersystem for intermittent first periods of triggered transmission ofpulses and intermittent second periods of locator frame operationbetween at least some of the first periods, said transmission pulses forthe first periods differing from the transmission pulses for the secondperiods in at least one of the following parameters: transmit power,transmit center frequency, transmit line spacing, receive centerfrequency, transmit bandwidth, transmit spectral shape.
 7. The system ofclaim 6 wherein transmit power is greater in the first periods than thesecond periods.
 8. The system of claim 6 wherein transmit line spacingis closer in the first periods than the second periods.
 9. The system ofclaim 1 wherein the transmit beamformer system is operable to acquiretriggered frames to enhance image quality and is operable to acquirelocator frames between the triggered frames to reduce contrast agentdegradation.